The present invention generally relates to an improvement of an operating circuit for a high-pressure discharge lamp, such as a metal halide lamp, high-pressure sodium lamp, high-pressure mercury lamp and others, which are operated by a D.C. or a high frequency power source. More particularly, the invention concerns a configuration of the discharge lamp operating circuit which is suited for assuring a stabilized output so that an arc spot is constantly produced at the tip of the electrode of the high-pressure discharge lamp.
The high-pressure discharge lamp, such as the metal halide lamp, the high-pressure sodium lamp, the high-pressure mercury lamp or the like, is usually operated or driven by an A.C. power source of a commercial frequency, such as 50 Hz, or 60 Hz because the use of the lamp has heretofore been limited to application for illumination. In recent years, however, the high-pressure discharge lamp tends to be used for measurements and industrial purposes. In most of these applications, it is preferred that no fluctuations be present in the light output. To this end, the discharge lamp is operated from a D.C. power source or a high-frequency power source of a frequency equal to or higher than 10 KHz in many applications. In that case, however, the arc spot on the electrode is frequently deviated from the normal position, involving adverse influence to the lamp characteristics such as intensity, stability, useful life and so on. FIGS. 1A and 1B are views for illustrating the position at which the electrode arc spot is produced in an arc tube of a metal halide lamp of 60 W taken as an example. In the figures, reference numerals 21 and 21' denote discharge plasmas, 22 denotes main electrodes made of tungsten or the like, and 23 and 23' denote the arc spots on the electrodes. FIG. 1A shows a normal discharge state in which the arc spots 23 are produced at the tips of the main electrodes 22, respectively, i.e. at the normal position. On the other hand, in the abnormal discharge state shown in FIG. 1B, the arc spots 23' are produced at the roots of the main electrodes 22, respectively, i.e. at the abnormal position. When the arc spot 23' is produced at the abnormal position, there arises a problem that the light intensity is unstable. Particularly when the arc spots 23' are produced on the roots of the electrodes 22 as shown in FIG. 1B, the sealed parts of the electrodes located at the respective ends of the arc tube are heated excessively, bringing about not only the problem of fluctuations in the light output but also a problem that the useful life of the arc tube is shortened.
As will be appreciated from the above description, the hitherto known discharge lamp operating circuit driven by a D.C. power or high-frequency power of a frequency not less than 10 KHz suffers serious shortcomings. Accordingly, there is a demand for elimination of these shortcomings.